1. Field of the Invention
This invention relates to cathode ray tube (CRT) imaging systems, and is addressed specifically to means for electrically energizing devices and systems designed to suppress or otherwise cancel stray magnetic fields emanating from such displays. The invention is applicable to monochrome and color CRT image displays used in monitors and television sets.
The present invention is in response to a concern over the possible detrimental effects of stray magnetic fields on the physiology of image display viewers. Testing for such fields in visual display terminals is described in a publication of the National Board for Measurement and Testing (MPR) of Sweden entitled "Test Methods for Visual Display Units: Visual Ergonomics and Emission Characteristics"; MPR 1990:8 1990-1991, Boras, Sweden. This standard is known as "MPR-2."
As is known, the primary source of stray magnetic fields in CRTs is the yoke. The yoke is an electromagnetic device that causes an electron beam (or "beams" in color CRTs) to scan a raster on the CRT viewing screen in both the horizontal and vertical directions. Essentially, a yoke consists of two pairs of coils, one of which deflects an electron beam in the horizontal direction, and the other in the vertical direction. The two pairs of coils appear as dual radiating magnetic dipoles. The respective deflection coils of the yoke are energized by a horizontal oscillator circuit and a vertical oscillator circuit. The horizontal oscillator circuit provides a train of sawtooth pulses having a frequency of 15,750 Hz in monochrome television sets, a frequency of 15,734.26 Hz in color television sets, and frequencies of up to 150 kHz in some visual display terminals. The pulses are routed to the electrical windings of the yoke that provide for the horizontal excursion of an electron beam across the imaging screen. A horizontal oscillator circuit typically emits a train of positive-going pulses having an amplitude of 400 to 1,000 volts. The pulses are integrated by the yoke circuit into a current having an amplitude in the range of three to twenty amperes for beam deflection.
Cost considerations require that horizontal oscillator circuits (and other circuits as well) be designed to meet the exact voltage and amperage requirements needed to drive the yoke, with no excess capacity. Hence the addition of stray magnetic field suppression devices that require substantial amounts of power has required a redesign of the horizontal oscillator circuits and in some cases, the power supplies, with consequent cost penalty.
In the context of this disclosure, the term "auxiliary" can be further defined as a "supplement"; that is, it supplements the role of a major circuit in a television system or monitor. In short, it is an adjunct that is not essential to the operation of the television set or monitor, but one that is used solely to energize a system for the suppression of the stray magnetic fields they generate. Essentially, the auxiliary pulse generator according to the invention is a substitute for the horizontal oscillator circuit, relieving the horizontal oscillator circuit from the need to energize a stray magnetic field suppression system. In consequence, the circuits of a CRT imaging system are not affected by the addition of such a system.
2. Discussion of Related Art
The present invention is concerned with means for energizing devices that provide for the suppression of the stray magnetic field that emanates from the horizontal deflection coil of the yoke. Such stray magnetic field suppression means include those disclosed in commonly owned referent copending applications Ser. No. 814,125 now U.S. Pat. No. 5,208,510 and Ser. No. 927,705. Essentially, stray magnetic fields are suppressed by interposing into the path of the stray field one or more electromagnetic devices that emit a radiation having a polarity opposite to that of the stray field, thus effectively cancelling the stray field.
Also, the means according to the present invention is effective in energizing the system for suppressing AC electric field emissions disclosed in referent copending application Ser. No. 868,922 now U.S. Pat. No. 5,231,332, also of common ownership.